neorados: Hold reference to implementation across operations

neorados: Hold reference to implementation across operations

Asynchrony combined with cancellations keeps leading to occasional
lifetime issues, so follow the best-practices of Asio I/O objects by
having completions keep a reference live.

The original NeoRados backing implements Asio's two-phase shutdown
properly.

The RadosClient backing does not, because it shares an Objecter with
completions that do not belong to it. In practice I don't think this
will matter since librados and neorados get shut down around the same
time.

Signed-off-by: Adam C. Emerson <aemerson@redhat.com>